High powered radio frequency (RF) power amplifiers typically comprise an RF amplifying circuit, a DC source and/or distribution circuit, a chassis, and heat sinks. The RF amplifying circuit generally consists of impedance matching circuitry and a power amplifier device (usually transistors), such that RF signals are amplified. The RF amplifying circuit is connected to the chassis, where the chassis provides an RF ground path and mechanical support for the RF amplifying circuit. When the power amplifier device is amplifying RF signals it produces a substantial amount of heat; RF amplifying circuits are approximately 55% efficient. The power level of RF amplifiers can be increased by paralleling RF amplifying circuits, thus creating even more heat. Heat sink extrusions or heat sink castings, mounted on the back of the chassis, are typically used to dissipate the heat. A difficulty arises in getting the heat from the power amplifier device, which is typically located within the chassis, to the externally mounted heat sink extrusions or heat sink castings.
One technique is to use low thermal resistant heat sink interface devices (typically cooper bars or blocks) to develop a heat transfer path from the power amplifier device to the heat sink extrusions or heat sink castings. This method requires appropriate hardware to assure that the interface between the power amplifier device, the copper bar, and the heat sink extrusion maintains a low thermal resistance. In addition to the parts required, the interfacing device has thermal resistance which limits the heat transferred from the power amplifier device to the heat sink. Another method is to mount the power amplifier device directly on the externally mounted heat sink extrusions and connect it to the appropriate circuitry located within the chassis by wires or printed circuit board connections. This method reduces the thermal resistance of the heat transfer path but may present serious RF interference problems including electromagnetic interference (EMI).
Another important design aspect of high powered RF power amplifiers is the grounding of the RF amplifying circuit and the DC source and/or distribution circuit. In most high powered RF amplifiers, both the RF amplifying circuit and the DC source and/or distribution circuit are located within the chassis and grounded to it. Having both circuits within the chassis is primarily dictated by the fact that the heat sink extrusions or heat sink castings are so large that there is no other place to put them. The grounding of the circuits is usually done through conductive standoffs which create several low impedance paths to the chassis. Generally, the more conductive standoffs there are, the lower the total impedance will be, thus creating a better ground path. However, the use of conductive standoffs may result in distributed ground impedances or various contact impedances between the circuit and the standoff. Variations in the ground impedance may produce circulating ground currents which can impede or destroy the amplifier's performance.
In addition to providing a low impedance ground connection, the high powered amplifier must be designed to limit the effects of the RF interferences. Most power amplifier circuits use RF shields to enclose the RF amplifying circuit to limit the radiation or interception of Rf interferences. This technique generally requires hardware to hold the shield in place and makes the internal packaging of the power amplifier difficult.
A 600 Watt, class C power amplifier incorporating the above mentioned design features has a total of about 240 parts and weighs about 80 pounds.
Some lower powered RF amplifiers, 450 Watts for a class B amplifier, 80 Watts for a class A amplifier, use internally mounted heat pipes to dissipate the heat generated by the power amplifier devices. This technique substantially reduces the overall weight, but it increases the internal ambient temperature, does not reduce the part count, and generally creates more internal EMI. By having the internal ambient temperature higher, higher grade temperature components may be required, thus increasing the cost.
A need exists for a high powered power amplifier that requires substantially fewer parts, that weighs substantially less than a power amplifier using heat sink extrusions or heat sink castings, that has an improved thermal connection between the power amplifier device and the heat sink, that has an improved method of reducing the RF interference effects, and that has an improved ground system.